docs: arXiv references page (#21450)

Since the LangChain based on many research papers, the LC documentation
has several references to the arXiv papers. It would be beneficial to
create a single page with all referenced papers.
PR:
1. Developed code to search the arXiv references in the LangChain
Documentation and the LangChain code base. Those references are included
in a newly generated documentation page.
2. Page is linked to the Docs menu.

Controversial:
1. The `arxiv_references` page is automatically generated. But this
generation now started only manually. It is not included in the doc
generation scripts. The reason for this is simple. I don't want to
mangle into the current documentation refactoring. If you think, we need
to regenerate this page in each build, let me know. Note: This script
has a dependency on the `arxiv` package.
2. The link for this page in the menu is not obvious.

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Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>